# Current Observation and Measurement

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• Last Post 17 May 2018
Chris posted this 12 April 2018

I know most of you already know, but this post is to help brings others up to speed:

Ohms Law, for electronics, takes Resistance (Ohms), Voltage (Volts), Current (Amps) and Power (watts) and with a set of equations gives a very accurate prediction of each and every quantity, only requiring two input quantity's.

A Current Sensing Resistor (CSR) uses the equation: Current ( I ) = Voltage ( V ) / Resistance ( R

Because the Voltage Drop over a Resistance can be measured very accurately, and using a small resistance so as to not impede the Circuit much, we can get a very good idea of the Current in the Circuit. In the following example, we need to keep in mind the Units, using milliOhms requires a correction as will be shown.

Using Ohms Law, the Current through a 1 Ohm Resistor when 1 Volt is applied across it, is 1 Ampere: I = V / R = 1 / 1 = 1

In the above example, the Values chosen are:

• Resistance = 1.1 Ohms
• Voltage = 1 Volt

So, the equation, again: I = V / R = 1 / 1.1 = 0.90909

In the above simulation we get: 09098

Note: We are measuring a Voltage Drop across a Resistor that is 1/10th of an Ohm! We need to account for this because this resistor is 1/10th of of an Ohm, for the Total Circuit Resistance.

A correction factor of 10 must be applied! We have an easy option however! Setting the Oscilloscope Probe to 10x can automatically account for this correction, giving us:

09098 x 10 = 0.9098

Which is correct up to three decimal places, where the simulation perhaps accounting for trace resistance, 0.0008 of an Amp difference. Also rounding may play a part in this figure.

Calculated Resistance: 1.09914 for the given Current: 0.9098

Measuring, or observing Current is really important! Its a key step in understanding what Coils are doing, an observing how they interact together! We have learnt this in The Mr Preva Experiment!

I have covered some of this before, but, the measurement Blocks I have built, are very valuable little devices!

My Metal Strip Through Hole Current Sensing Resistors are 0.1 Ohms ±1% and have very little Inductance.

The following video is very good, but has a lot of silliness, I don't recommend using high Voltage or Hi Current, please be careful!

Also, watch wire length, no matter weather it is "Non-Inductive" it can still create problems!

Chris

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Jagau posted this 12 April 2018

How to learn electronics with a smile.

Very relaxing video!

Jack

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Chris posted this 17 May 2018

For all that are new and wish to learn more, when I build my measurement blocks, I built three, two of the ones above and one with a dual Current Sensing Resistors on it.

This is what I used in my Thread: Some Coils Buck and some Coils DONT

The basic Circuit:

You can see, this is only for Current, I am not taking any voltage measurement here.

I hope this helps some!

Chris

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The great Nikola Tesla:

Ere many generations pass, our machinery will be driven by a power obtainable at any point of the universe. This idea is not novel. Men have been led to it long ago go by instinct or reason. It has been expressed in many ways, and in many places, in the history of old and new. We find it in the delightful myth of Antheus, who drives power from the earth; we find it among the subtle speculations of one of your splendid mathematicians, and in many hints and statements of thinkers of the present time. Throughout space there is energy. Is this energy static or kinetic? If static, our hopes are in vain; if kinetic - and this we know it is for certain - then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature.

Experiments With Alternate Currents Of High Potential And High Frequency (February 1892).